6.2.2 Stars

Learning Objectives

Core

1. State that:

1. galaxies are each made up of many billions of stars
2. the Sun is a star in the galaxy known as the Milky Way
3. other stars that make up the Milky Way are much further away from the Earth than the Sun is from the Earth
4. astronomical distances can be measured in light-years, where one light-year is the distance travelled in (the vacuum of) space by light in one year

Supplement

2. Know that one light-year is equal to 9.5 × 10¹⁵ m
3. Describe the life cycle of a star:

1. a star is formed from interstellar clouds of gas and dust that contain hydrogen
2. a protostar is an interstellar cloud collapsing and increasing in temperature as a result of its internal gravitational attraction
3. a protostar becomes a stable star when the inward force of gravitational attraction is balanced by an outward force due to the high temperature in the centre of the star
4. all stars eventually run out of hydrogen as fuel for the nuclear reaction
5. most stars expand to form red giants and more massive stars expand to form red supergiants when most of the hydrogen in the centre of the star has been converted to helium
6. a red giant from a less massive star forms a planetary nebula with a white dwarf star at its centre
7. a red supergiant explodes as a supernova, forming a nebula containing hydrogen and new heavier elements, leaving behind a neutron star or a black hole at its centre
8. the nebula from a supernova may form new stars with orbiting planets

Contents to learn

The Milky Way galaxy

• The Milky Way is one of billions of galaxies that make up the universe and our Solar System is located within one of the spiral arms of the Milky Way.
• Galaxies are made up of billions of stars.
• The Sun is one of billions of stars in a galaxy called the Milky Way and other stars are much farther away from Earth than the Sun.
• Some stars also have planets that orbit around them.
• After the Sun, the star closest to Earth is Proxima Centauri but it is approximately 40 trillion kilometers away and it takes 4 light years to reach Earth.

Distance in Space – Light Year

• The speed of light is constant i.e.
• The outer region of the solar system is far from the Sun, and light takes some time to travel these distances.

• Light takes 8 minutes to reach Earth from the Sun and distance between the earth and Sun is approximately 1.5 x 10⁸km.
• Due to the vast distances in space, astronomers frequently use the light-year as a unit of distance.
• One light-year is the distance that light travels in one year in a vacuum, it is approximately 9.5 x  10¹⁵ m.
• Speed of light = 300,000 km/s
• 1 year = 365 days
• 1 day = 24 hours
• 1 hour = 60 minutes
• 1 minute = 60 seconds
• Distance = speed x time
• s =vt
• 1 light year = 300,000 x 365 x 24 x 60 x 60
• 1 light year = 9.5 x 10¹² km

The Life Cycle of a Star

A star is formed from interstellar clouds of hydrogen gas and dust called a nebula and it involves several key steps.

Proto star

• A proto star is an interstellar cloud of gas and dust collapsing, due to its internal gravitational attraction.
• Strong gravitational attraction continues to attract more gas and dust to increasing the temperature and pressure of the core.
• Strong gravitational attraction also squeezes the proto star, making it denser, and particles inside it collide more vigorously, causing its core temperature to increase.
• Due to the increase in temperature to about 15 million degrees Celsius, nuclear fusion reactions occur within the core of a protostar, releasing a large amount of energy.

• Hydrogen nuclei fuse to form helium nuclei, releasing a large amount of energy that keeps the core hot.

Main Sequence star

• A protostar becomes a stable star when equilibrium is achieved, where the inward force of gravitational attraction is balanced by the outward force caused by the high temperature resulting from nuclear fusion reactions in the core of the star.

• This stable star is known as a main sequence star and can remain in this stage for billions of years. Our sun has been in this stage for 4.6 billion years and will remain there for an estimated 6.5 billion years.
• Stars live out the majority of their lives in a main sequence phase.

Red Giant/Red Supergiant

• When the stable star ages, it gradually converts hydrogen into helium through nuclear fusion reactions, and hydrogen is exhausted.
• Due to the decline of hydrogen in the core, the nuclear fusion reaction stops, and equilibrium is not maintained.
• As a result, inward gravitational attraction takes over, causing the core to contract and heat up, increasing the pressure and temperature at its center.
• When the core contracts and heats up, hydrogen fusion begins in a shell surrounding the core and releases energy, causing the outer layers of the star to expand outward.
• This expansion leads to the formation of a red giant or a red supergiant depending on the initial size of the star.

1.     When it is a small to medium star like our sun, it will expand to form a red giant.
2.      When it is a massive star, it will expand to form a red supergiant.

• When the core temperature becomes high enough due to inward gravitational compression, helium fusion begins in the core, converting helium into heavier elements including all the elements up to iron in the periodic table.
• The expansion of the outer layers leads to the creation of strong stellar winds that blow heavy elements away from the star’s surface.

Life Cycle of a Star